1. Field of the Invention
This invention relates to the field of micro-electromechanical (MEM) devices, and particularly to the use of a liquid medium to improve the mechanical and electrical performance of such devices.
2. Description of the Related Art
Micro-electromechanical (MEM) devices feature at least one member which is movable with respect to another member which is typically fixed. Movement of the movable member is typically effected via electrostatic force: appropriate voltages are applied to both the movable and fixed members such that the resulting electrostatic force drives the movable member in a desired direction.
Such devices are subject to a number of problems. For example, the device""s response time can be degraded due to mechanical xe2x80x9cringingxe2x80x9d that can occur when a movable member is actuated such that it moves from one position to another. In addition, the device""s sensitivity to external mechanical vibrations can limit its usefulness to low-g environments.
One type of MEM device is a tunable capacitor. Capacitance is tuned by controlling the overlap of the capacitor plates (which may be parallel plates or interdigitated fingers). This may be accomplished by coupling one of the plates to a movable member which includes a set of xe2x80x9cfingersxe2x80x9d, which are interdigitated with a set of fingers on a fixed member. Movement is effected by applying a drive voltage across the fixed and movable members, creating an electrostatic force which moves the movable memberxe2x80x94and thus the capacitor platesxe2x80x94in a desired direction.
MEM tunable capacitors suffer from several drawbacks, however. Constraints on device size typically limit the achievable capacitance values to the 1-10 pF range. High drive voltages (up to 35-40 volts) are often necessary to obtain the achievable capacitance range. Furthermore, the time required to achieve a desired capacitance value may be unacceptably long, due to the mechanical ringing noted above.
A known MEM current sensor is similarly affected. A current to be measured is used to drive the movable member of a tunable capacitor as described above, and the resulting capacitance is sensed to set an analog signal or digital output bits which represent the sensed current value. Unfortunately, the problems noted above also impact this device: current sensing range is limited due to device size constraints and capacitance value limitations, and response time may be unacceptably long. In addition, existing designsxe2x80x94which operate in airxe2x80x94have limited heat dissipation capabilities which might limit the maximum current that can be sensed. Furthermore, the sensor""s current-carrying member may need to be electrically isolated from the sensor""s other structures to prevent breakdown; this isolation can result in a device which is unacceptably large.
MEM devices having their fixed and movable members immersed in a liquid medium are presented. When the liquid is properly selected, the devices"" mechanical and electrical performance are improved.
A MEM device in accordance with the present invention includes at least one member capable of moving with respect to a second, typically fixed member. Movement is effected with a drive voltage or an applied current which creates a forcexe2x80x94such as an electrostatic, Lorentz, or thermal forcexe2x80x94that causes the movable member to move with respect to the fixed member. The movable and fixed members are immersed in a liquid medium having particular characteristics. The liquid medium is preferably selected to have a viscosity which critically damps the motion of the movable members, thereby improving the device""s mechanical performance. The liquid may also be chosen to provide a dielectric constant greater than one, such that, where applicable, the device""s maximum capacitance and capacitance range are increased, and the electrostatic force created for a given drive voltage is increased over what it would be in airxe2x80x94thereby improving the device""s electrical performance. The liquid medium might also be used to improve the device""s thermal dissipation characteristics, or to provide improved isolation between the different structures which make up the device.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.